Pregnant sheep will be studied chronically during the last month of gestation to determine normal relationships between fetal growth, rate of O2 metabolism, rate of uterine blood flow and several maternal factors which may limit availability of O2. Particular attention will be given to the relationship of these variables to blood concentrations of endogenous steroid hormones and the role they may play in the regulation of these processes. Additional observations will be made when pregnant ewes are stressed by heat and by limited surface area of the uterine lumen.